The present invention relates to a technology effective for application to a method of setting redundancy relieving information and trimming information such as a voltage employed in an electrically programmable erasable non-volatile memory, and to a technology effective for use in a flash memory, for example.
In a flash memory, nonvolatile storage or memory elements comprising MOSFETs formed in a two-layer structure having control and floating gates are used for memory cells. The amount of an electrical charge stored in the floating gate is changed to vary the threshold voltage of each MOSFET, thereby storing data in each memory cell.
This type of flash memory is generally provided with an internal power circuit having a booster circuit like a charge pump circuit for generating high voltages necessary for write/erase operations for each memory cell. However, the booster circuit causes predetermined variations even in the generated voltages due to variations in elements constituting the booster circuit. Even as to the MOSFETs constituting the storage elements of the flash memory, parameters such as the thickness or the like of a gate oxide film, the size of each portion of an element, the concentration of an impurity in a drain region, etc. vary due to the difference in process or the like, and correspondingly, a write characteristic and an erase characteristic vary in a predetermined range.
When the voltages generated by the booster circuit and the write and erase characteristics of each storage element vary as described above, the accurate operation of the memory is not assured. Therefore, there is known a technology wherein a trimming circuit is provided to make fine adjustments to each generated time and a write time at a stage subsequent to the fabrication of a chip. A general semiconductor memory including a flash memory is provided with a so-called redundant circuit for replacing a defective bit included in a memory array with its corresponding spare memory cell to improve the yield thereof.
It was conventionally common practice to adopt a system wherein the level setting of the trimming circuit and the setting of substitutional information by the redundant circuit were carried out by using a fuse (hereinafter called “polysilicon fuse”) formed of a polysilicon layer. However, the system using the polysilicon fuse needs a device for breaking or cutting off the polysilicon fuse by laser or the like. Since the subsequent change is unfeasible once it cuts off, the greatest possible care is required upon its cutting-off. A problem also arises in that trimming cannot be performed after the assembly of chips into packages. Therefore, there has also been proposed the invention related to a trimming circuit or a redundant circuit wherein elements identical in structure to nonvolatile storage or memory elements constituting a memory array, which are used in place of the polysilicon fuse, have been used in place of the polysilicon fuse.